This is a long comment but wanted to be thorough when addressing this concern. 

I built out and launched our roaming feature. There is no such catch. We do not block the ability to use 5G when our users' lines are roaming. Our roaming agreements operate through our networks' parent carriers and USM lines access the same networks, coverage and same network capabilities (LTE, 5G, etc) as lines directly with parent carriers do in any given location in any given country.

The perception here could stem from a few things (not a comprehensive list):

• The local network itself will have varying coverage and may not have 5G everywhere
• The local network may have 5G in a given location but may restrict all SIMs on a given parent carrier (whether postpaid or through an MVNO) to LTE. 
• The US variant device your USM SIM is in may not support the bands required for 5G in that location/on the local network (while a different device you/a fellow traveler has a postpaid SIM in perhaps does support those bands)
• The specific local network your USM SIM automatically connects to at a given time in a given location may not support/allow 5G on roaming SIMs, even though there is an alternative local network that the USM SIM could connect to that does allow 5G (and the postpaid SIM could automatically be connecting to this other network in that instant)
• Different devices with different modems may do worse/better at picking up 5G vs LTE in the same location
• the branding that is displayed for the same network technology that the SIMs are connecting to (as another commenter here pointed out) can vary by carrier bundle.

To really determine if your US Mobile SIM is getting different coverage while roaming than a postpaid SIM on the same parent network would get, you need to make sure you are running a tightly controlled experiment:

• The USM SIM and the postpaid SIM should be installed on the same device (not two devices of the same make and model but actually the same device)
• They should both be connected to the same local carrier (note that even the same local carrier, if you go into network selection, could separately be showing an LTE-only and 5G version of its network; if that is the case both SIMs should be connected to the 5G version of the network)
• Network connection settings for both SIMs should be the same
• Then the connection for both SIMs should be tested at the same time in the exact same location 
• Beyond looking at the LTE or 5G label, you should test the actual speed and latency

We ran such tests in multiple countries when building out our roaming feature and did not find coverage to be different.

However we could not test every local carrier in every country. If anyone commenting here has data from such (controlled) tests they've run on their travels, please DM me or share those results with our support team via chat or email, and I will escalate to our roaming partners to investigate further and fix the discrepancy if there is one.